There is an increasing need for storage media with very high volumetric storage density, high information input rate capability, and stored information stability considered "archival". Optical tape promises to address all of these needs.
The competing technology, magnetic recording, does not yet meet the volumetric storage density needs which optical tape can provide now and is not perceived as archival at an acceptable level.
Attempts to make optical tape have typically evolved from efforts related to other optical storage elements, typically on rigid supports, such as optical cards or optical disks. Most of these elements have utilized dye layers to absorb light, with deformation or ablation of one or a sandwich of layers being the mechanism for changing the reflectance or transmittance to effect digital information storage. Others have utilized the deformation of metal or alloy layers or the flow of low melting point metals or alloys upon the absorption of optical energy. Still others rely on the rotation of the polarization of light from areas of a special alloy layer heated and cooled in the presence of appropriate magnetic fields (magnetooptic recording); or the change in phase of a metal or alloy from amorphous to crystalline or between crystalline phases upon local heating accompanied by a detectable change in the reflectance (or transmittance) from (or through) the areas of differing crystal phases.
The approaches involving the ablation or movement of material to form pits or deformed areas of differing reflectance have generally required the application of overcoats or encapsulation layers to contain the removed material or byproducts. This has resulted in reduced sensitivity and/or resolution (spot sharpness).
In Japanese Kokai JP04/163,736 assigned to Mitsubishi Kasei, there is disclosed an optical tape having a phase-change optical recording layer or a magnetooptic recording layer and further having a protective and interference and/or reflection layer.
One particularly advantageous optical recording material is the phase-change optical recording SbInSn alloys described in commonly assigned U.S. Pat. No. 4,904,577, 4,960,680 and 5,271,978. This material is characterized by having good writing sensitivity, good signal-to-noise ratio, wide wavelength response and good resolution. It has also been found to be compatible with coating on a flexible support needed for flexible digital optical tape. The recording material in the last mentioned patent, '978 patent include a dopant which improves the stability to mark change on storage and is the currently preferred material.
However, these materials are relatively soft alloys, thus, relatively prone to scratching. In many of the optical disk applications (non-flexible substrates) scratch resistance can be provided by applying relatively thick overcoats, laminates, or even coverplates spaced well away from the recording layer surface. In the case of flexible digital optical tape which is wound upon itself, the need for high volumetric storage density requires that the protective layer be thin, thus also requiring that it be an integral part of the optical-thin-film package. While a wide variety of materials have been suggested for other types of recording materials, it was not clear which overcoat candidates would have the necessary combination of adhesiveness to the phase-change SbInSn alloy, toughness, flexibility and other properties that are required.
One of the more difficult problems to overcome for this type of material is corrosion resistance since the materials are intended for archival keeping. One of the common tests that is used to determine the corrosion resistance is the Battelle Class II corrosion test. This is described, for example, in Br. Corros. J., 24 (2), p 153 (1989). In this test, a sample is exposed to an environment containing 70% relative humidity and a variety of corrosive gasses such as chlorine, hydrogen sulfide and nitrogen oxide at low concentrations. The test involves exposing the test sample to flowing gasses for extended periods. It is commonly assumed that the acceleration factor is about 180:1. Thus, 30 days in this test represents about 15 years of normal storage.
A wide variety of potential overcoats for this type of element are known. It is important that the overcoat provides all of the physical properties mentioned, e.g. adhesion, while at the same time, excellent corrosion resistance in the Battelle Class II test. It is to the solution of this problem that the present invention is directed.